Nonsynaptic localization of the excitatory amino acid transporter 4 in photoreceptors

Short term optical defocus perturbs normal developmental shifts in retina/RPE protein abundance

BACKGROUND

Myopia (short-sightedness) affects approximately 1.4 billion people worldwide, and prevalence is increasing. Animal models induced by defocusing lenses show striking similarity with human myopia in terms of morphology and the implicated genetic pathways. Less is known about proteome changes in animals. Thus, the present study aimed to improve understanding of protein pathway responses to lens defocus, with an emphasis on relating expression changes to no lens control development and identifying bidirectional and/or distinct pathways across myopia and hyperopia (long-sightedness) models.

RESULTS

Quantitative label-free proteomics and gene set enrichment analysis (GSEA) were used to examine protein pathway expression in the retina/RPE of chicks following 6 h and 48 h of myopia induction with - 10 dioptre (D) lenses, hyperopia induction with +10D lenses, or normal no lens rearing. Seventy-one pathways linked to cell development and neuronal maturation were differentially enriched between 6 and 48 h in no lens chicks. The majority of these normal developmental changes were disrupted by lens-wear (47 of 71 pathways), however, only 11 pathways displayed distinct expression profiles across the lens conditions. Most notably, negative lens-wear induced up-regulation of proteins involved in ATP-driven ion transport, calcium homeostasis, and GABA signalling between 6 and 48 h, while the same proteins were down-regulated over time in normally developing chicks. Glutamate and bicarbonate/chloride transporters were also down-regulated over time in normally developing chicks, and positive lens-wear inhibited this down-regulation.

CONCLUSIONS

The chick retina/RPE proteome undergoes extensive pathway expression shifts during normal development. Most of these pathways are further disrupted by lens-wear. The identified expression patterns suggest close interactions between neurotransmission (as exemplified by increased GABA receptor and synaptic protein expression), cellular ion homeostasis, and associated energy resources during myopia induction. We have also provided novel evidence for changes to SLC-mediated transmembrane transport during hyperopia induction, with potential implications for signalling at the photoreceptor-bipolar synapse. These findings reflect a key role for perturbed neurotransmission and ionic homeostasis in optically-induced refractive errors, and are predicted by our Retinal Ion Driven Efflux (RIDE) model.

Inositol 1,4,5-trisphosphate signaling maintains the activity of glutamate uptake in Bergmann glia

The maintenance of synaptic functions is essential for neuronal information processing in the adult brain. Astrocytes express glutamate transporters that rapidly remove glutamate from the extracellular space and they play a critical role in the precise operation of glutamatergic transmission. However, how the glutamate clearance function of astrocytes is maintained remains elusive. Here, we describe a maintenance mechanism for the glutamate uptake capacity of Bergmann glial cells (BGs) in the cerebellum. When inositol 1,4,5-trisphosphate (IP(3) ) signaling was chronically and selectively inhibited in BGs in vivo, the retention time of glutamate around parallel fiber-Purkinje cell synapses was increased. Under these conditions, a decrease in the level of the glutamate/aspartate transporter (GLAST) in BGs was observed. The same effects were observed after chronic in vivo inhibition of purinergic P2 receptors in the cerebellar cortex. These results suggest that the IP(3) signaling cascade is involved in regulating GLAST levels in BGs to maintain glutamate clearance in the mature cerebellum.

Glutamate, excitatory amino acid transporters, Xc- antiporter, glutamine synthetase, and gamma-glutamyltranspeptidase in human corneal epithelium

PURPOSE

The viability and functions of the corneal epithelium are dependent in large measure on the active uptake of nutrients, growth factors, and amino acids from stroma and tear. The present study presents the cellular distribution(s) of glutamate, the Na(+)-dependent glutamate/aspartate transporters (excitatory amino acid transporters; EAAT1-5), Na(+)-independent glutamate/cystine exchanger (Xc(-) antiporter) subunits (xCT light chain and 4F2hc heavy chain), glutamine synthetase (GS), and gamma-glutamyltranspeptidase (GGT) in human corneal epithelium.

METHODS

Glutamate, EAAT1-5, xCT/4F2hc, GS, and GGT immunoreactive proteins were detected by immunofluorescence microscopy. Human corneal GGT activity was quantified using a standard colorimetric assay.

RESULTS

Glutamate, EAAT3>2>1, xCT/4F2hc, and GGT proteins were detected in the columnar and wing cells. Glutamate was reduced or absent in the EAAT negative, Xc(-) antiporter, and GS positive outer wing cell and flat superficial epithelial cell layers. All EAATs (EAAT3>4/5>1/2), xCT/4F2hc, GS, and GGT were detected in flat superficial epithelial cell layer.

CONCLUSIONS

The localization of glutamate, multiple EAATs, Xc(-) antiporter proteins, and GGT to columnar and superficial epithelial cell layers suggests uptake of glutamate and cystine from the stroma and tear and supports their importance in regulation of glutamate/cystine and glutathione (GSH; a tripeptide of glutamate, cystine, and glycine) in the human cornea epithelium. In addition, the low glutamate levels in outer wing and flat superficial epithelial cells positive for Xc(-) antiporter and GS are consistent with exchange of glutamate by Xc(-) antiporter for extracellular cystine utilized in GSH synthesis and support coupling of ammonia detoxification with glutamate degradation by GS.

Ankyrin-B is required for coordinated expression of beta-2-spectrin, the Na/K-ATPase and the Na/Ca exchanger in the inner segment of rod photoreceptors

Rod photoreceptors are highly polarized cells whose exquisite sensitivity to light depends on precise compartmentalization of ion channels/transporters within specialized membrane domains. Here, we report evidence for an ankyrin-B based mechanism for coordinated expression of the beta-2-spectrin-based membrane skeleton, and the Na/K-ATPase and Na/Ca exchanger in the inner segment of rod photoreceptors. We first discovered that ankyrin-B localizes to the inner segments but not outer segments of rod photoreceptors in vertebrates including humans, mice, and frogs. We found that haploinsufficiency of ankyrin-B in mice is accompanied by 50% reduction in inner segments of membrane proteins, including the Na/K-ATPase and the Na/Ca exchanger, as well as beta-2-spectrin, which is a component of the spectrin-actin membrane skeleton. These results are consistent with a mechanism where ankyrin-B is required to restrict the Na/K-ATPase and Na/Ca exchanger to the inner segment of rod photoreceptors by tethering these membrane proteins to beta-2-spectrin.